This reverts commit 8de2e6db.

Reason for revert: Suspect for:
https://build.chromium.org/p/client.v8/builders/V8%20Win32%20-%20nosnap%20-%20shared/builds/24237
https://build.chromium.org/p/client.v8/builders/V8%20Win64/builds/20896

Original change's description:
> [platform] Implement TaskRunners in the DefaultPlatform
> 
> This CL implements the TaskRunners in the DefaultPlatform which has been
> added recently to the platform API. In addition I changed how task
> posting works on the DefaultPlatform.
> 
> With this implementation the DefaultPlatform keeps one
> DefaultForegroundTaskRunner per isolate, plus one
> DefaultBackgroundTaskRunner. The DefaultPlatform owns these TaskRunners
> with a shared_ptr, which is also shared with any caller of
> GetForegroundTaskRunner or GetBackgroundTaskrunner.
> 
> This CL moves the task management from the DefaultPlatform to the
> TaskRunners.  The DefaultForegroundTaskRunner owns and manages the the
> task queue, the delayed task  queue, and the idle task queue. The
> DefaultBackgroundTaskRunner owns the WorkerThread pool and the
> background task queue.
> 
> In addition changed many Task* to std::unique_ptr<Task> to document task
> ownership.
> 
> R=​rmcilroy@chromium.org
> 
> Change-Id: Ib9a01f1f45e5b48844a37d801f884210ec3f6c27
> Reviewed-on: https://chromium-review.googlesource.com/753583
> Commit-Queue: Andreas Haas <ahaas@chromium.org>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49354}

TBR=rmcilroy@chromium.org,ahaas@chromium.org

Change-Id: I6c381915a2cae8ea1a0d21d6cfa6e797ccd2d947
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/768748Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49357}